The present invention relates to an RF ground plane choke for strip transmission line media and, more particularly, to a choke which provides for a noncontacting RF ground plane to ground plane connection at an interface without introducing discontinuities that adversely affect VSWR (voltage standing wave ratio) and which allow relative movement between the strip transmission line substrate and supporting metal carrier due to differences in thermal expansion or contraction. The term "strip transmission line" as used herein refers to a transmission line medium such as strip line, or microstrip line, or similar media, in contrast to coaxial media.
Heretofore, RF ground plane to RF ground plane connection between strip transmission line alumina substrates at an interface has been made by attaching the ground plane to a metal carrier, chassis or container by using solder or a conductive epoxy cement. However, a solder connection fails when subjected to a low shear stress generated by differences in thermal expansion or contraction between the substrate and metallic ground. If smaller substrates are used to reduce the differences in the thermal expansion or contraction, then more sections, and thus more interconnects, are needed to cover the same area. However, increasing the number of interconnects degrades the electrical performance while at the same time lowering the yield and reliability of the assembly. Bonding by use of conductive epoxy cement does not provide an adequate low-loss RF ground connection. Furthermore, attachment by means of solder or conductive epoxy involves many critical production steps that are not easily carried out.
As an alternative, a soft board material (Teflon ceramic composite) has been used in place of the alumina substrate, the soft board material having been pre-laminated to a thick aluminum metal plate for grounding purposes. However, it is difficult to establish a reliable, low-loss short circuit connection from the strip transmission line circuit side to the ground plane using the softboard laminate because of dissimilar metals of the aluminum ground plane and circuit lines consisting of either copper or gold. The soft-board laminate can be obtained with a copper metal plate for grounding, but it is expensive and very heavy. Additionally, it is difficult to make a good wire bond to the circuit side of soft board laminate because it yields easily when subjected to the compressive forces of thermal compression or thermosonic wire bonding. Also, the soft-board composite can be obtained with a thin copper ground plane rather than a thick metal plate for grounding, but the softboard warps after the circuit has been etched on one side. These disadvantages plus the high cost, heavy weight and high hygroscopicity of laminated soft board make it a poor choice in comparison to an alumina substrate.
Employment of a virtual short circuit instead of metal-to-metal contact to provide an RF ground plane to RF ground plane connection in a microwave integrated circuit is disclosed in U.S. Pat. No. 4,611,186 to Ziegner entitled "Noncontacting MIC Ground Plane Coupling Using A Broadband Virtual Short Circuit Gap". However, the patent employs a two-part half wavelength waveguide transformer formed by shaped cavities in a metal housing that is not adaptable for use in allowing relative movement between the strip transmission line substrate and metal carrier due to differences in thermal expansion or contraction.
Accordingly, it is a feature of the present invention to provide a low-loss RF ground plane to RF ground plane connection in a strip transmission line medium without using metal-to-metal contact.
Another feature of the invention is the provision of an RF ground plane to RF ground plane connection in a strip transmission line medium that allows for relative movement due to differences in thermal expansion or contraction between the strip transmission line substrate and the metal carrier.
A further feature of the present invention is to provide a low-loss RF ground plane to ground plane connection in a strip transmission line medium that is not rigidly bonded using solder, or the like.